The present invention relates to a compressor for use in an air-conditioner, a refrigerator or the like and, more particularly, to a technique of combining the materials of ferrous or iron-based sliding parts so as to provide them with abrasion-resistance and improved economic efficiency and thus allow them to be suitably used for high-performance, high-reliability rotary compressors.
Among the compressors that are used for air-conditioners, refrigerators and the like, there are various types including the rotary type, reciprocating type, scroll type and screw type. An explanation of the rotary type will be given here as a typical example of such compressors. A rotary compressor comprises a crankshaft rotatably supported by an upper bearing and a lower bearing, a roller rotatably mounted on a crank pin of the crankshaft, a cylinder for accommodating the roller, and a vane disposed slidably within a vane slot formed in the cylinder. A distal end portion of the vane is disposed in slidable engagement with an outer peripheral surface of the roller. The afore-mentioned sliding members of the compressor operate to compress Freon gas under the condition in which lubricant oil with Freon gas dissolved therein provides lubrication. Since these sliding members require an appropriately lubricated condition and an abrasion-resistance, the conventional compressor is designed, in general, to have iron-based sliding members, such as upper and lower bearings made of flake graphite cast iron or an iron-based sintered material, a crankshaft made of eutectic graphite cast iron, spheroidal graphite cast iron or flake graphite cast iron, a vane made of high-speed tool steel, and a cylinder formed from eutectic graphite cast iron or an iron-based sintered material.
However, in compact and high-output compressors of the variable speed control type which are capable of coping with the current tendency of air-conditioners and refrigerators to be provided with sophisticated functions and which employ such combination of the above-mentioned sliding members, a so-called boundary lubrication region in which metal-to-metal contact takes place due to break of oil film occurs during low-speed operations under high load and quick starting operations and in the operating condition in which the lubricating oil film is formed by low-viscosity refrigerator oil diluted with Freon. Consequently, since both the coefficient of friction and the abrasion loss increase, and since abrasion particles and fine foreign substances entering during fabrication accelerate the break of an oil film, the compressors tend to fall short of providing satisfaction in respect of the mechanical performances and the reliabilities.
To overcome the problem, the following several proposals have been made with a view to strengthening abrasion-resistance, but they each have their advantages and disadvantages and none of them are based on optimum combinations of materials that offer excellence in terms of both abrasion-resistance and productivity:
Japanese Patent Examined Publication No. 55-4958 discloses a rotary compressor which employs a combination of a cast iron cylinder and a soft-nitrided iron-based sintered alloy for the roller and the vane or for either one of them. In the soft-nitriding of a porous iron-based sintered alloy, nitriding reaction proceeds preferentially into the pores, while a change in configuration occurs to a large extent. This leads to the problem that it is difficult to effect reworking for the purposes of dimensional adjustment and removal of salt-bath components adhering to the alloy. Furthermore, since the pores and nitrides produce notch effect and the fatigue strength and mechanical strength are both low, such a combination of materials is inappropriate for driving parts to be employed in a compact, high-output and high-performance compressor.
Japanese Patent Unexamined Publication No. 60-73082 proposes a compressor which is characterized in that the internal surface of the cylinder is formed of an iron-based sintered alloy containing from 10 to 40 volume % of iron-based oxides, while the rotor and/or the vane is formed of an iron-based sintered alloy in which carbides of metal and oxides of metal are dispersed in a matrix formed by annealing martensite, and nitrogen is solidly dissolved in the matrix. Furthermore, the alloy component is restricted to iron, chromium, carbon, nickel, copper, or molybdenum and the material is sintered. However, as the constituent materials of a vane for use in a compact compressor with sophisticated functions that is required to have high output and high performance, these materials are appreciably inferior to conventional materials formed of molten metals in respect of mechanical strength and fatigue strength, as in the case of the aforementioned example.
Japanese Patent Unexamined Publication No. 62-13784 discloses a proposal in which a crankshaft is immersed in a salt bath mainly composed of a cyanate alkali metal salt, and a porous layer of iron nitride containing iron sulfide is formed thereon while a layer of an alloy formed of iron nitride is formed inside the porous layer. However, salt bath components of a high toxicity are liable to enter the porous portions and the oil ports in the crankshaft. Hence, unwashed portions of these components are liable to remain, and these components permeate the graphite contained in the cast iron, thus unfavorably resulting in blooming after treatment. Consequently, a process of strengthened washing is required, which necessitates a treatment for making non-polluting the waste solution left after washing. These processes appreciably adversely affect productivity and economic efficiency. In addition, in the cases where the treatment with salt bath and nitriding is effected, the surface-roughness of the crankshaft becomes relatively large, there is a further drawback that, with respect to precision parts such as compressor components which require control of dimensional accuracy on the order of several micron meters, reworking is required after processing so as to ensure dimensional accuracy. With respect to thermal stability of a combination of sliding members exposed to Freon and refrigerator oil, the iron sulfide component of the sulfided and nitrided layer is dissolved by its reaction with hydrochloric acid which is a decomposition product of Freon. Hence, there has been the drawback that this material is not suitable for use in high temperature environments.
For these reasons, there has been no optimum combination of sliding members available which offers a satisfactory performance for compact, high-output and high-performance compressors.
As described above, in the prior art, any proper consideration has not been given to the overall aspects of providing high-strength sliding parts for driving a compact, high-output and high-performance compressor, including satisfactory mechanical strength, adequate oil retaining properties, affinity to coexisting components, abrasion-resistance, production efficiency in terms of the need for complicated post-processings such as the washing-away of adhering salt-bath components and dimensional finishing, and so forth. Thus, in the field of compressors using Freon, there have been various problems in regard to the mechanical performance, such as the need for high-strength sliding materials for a compressor of a compact size and high performance, the need to minimize mechanical loss and maximize the volumetric efficiency of the compressor, as well as a demand for improvements in the long-term operational reliability and for a lowering of production costs and so forth.